A conventional terrestrial cellular frequency re-use scheme is illustrated in FIGS. 1 and 2. The available frequency spectrum is partitioned into frequency blocks f1 to f7 as shown in FIG. 2, each of which is assigned to a corresponding cell in a repeating cell cluster, as shown in FIG. 1. In this specific example, a hexagonal 7-cell cluster is used. The distance d in FIG. 1 represents the re-use distance of the cluster, in other words the centre-to-centre distance between cells to which the same frequency can be assigned. In this case, d=√{square root over (7)}D where D is separation between centres of adjacent cells. In any cellular re-use system, the minimum re-use distance is chosen so that the interference between channels assigned the same frequency in different cells is kept below an acceptable threshold.
The re-use factor of a particular cellular re-use scheme can be defined as the number of cells in which a particular carrier can be used, divided by the total number of cells. In the conventional scheme described above, the re-use factor is 1/7. Other schemes may use 3, 4, 19 or some other number of cells per cluster, depending on the desired re-use distance. The conventional scheme is designed to maximize this re-use factor.
The conventional re-use scheme described above does not provide efficient use of carriers where the demand for carriers is uneven across the cell pattern. For example, there may be a demand for more carriers in one cell than in other cells within a cluster. However, it is not possible to allocate more carriers to that one cell, while satisfying the minimum re-use distance requirement, without making the same allocation of carriers to the corresponding cell in all the other clusters.
In cellular communications systems, traffic demand distributions vary according to various temporal and spatial factors such as time of day, distribution of terminals, type of communication, and one-off events. For example, in a beam pattern which extends across different countries and time zones, as is common with satellite beam patterns for example, peak traffic demand will occur at different times and different levels in different beams.
These problems can be overcome to some extent by dynamic carrier assignment algorithms, for example temporary ‘borrowing’ of carriers for one cell experiencing a high demand from an adjacent cell having a low demand, without violating the minimum re-use distance requirement. However, borrowing is spectrally inefficient and it is usually not possible or desirable to coordinate the borrowing of carriers across multiple cells. Alternatively or additionally, asymmetric reuse schemes can be devised.
The document U.S. Pat. No. 6,269,245 discloses a carrier allocation scheme for a satellite cellular system, in which a 7-cell cluster pattern is imposed on the beams. Each cell is assigned a demand value representing the demand for carriers in each cell. For each cell in turn, a 19-cell re-use zone is centred on that cell and the maximum demand values within the re-use zone are reduced until the total demand within the re-use zone is no greater than the total capacity of the satellite system. Next, a static allocation scheme is constructed by dividing the available channels into a ‘preferred channel’ set comprising three pools: a base demand pool for satisfying the minimum channel requirements of the cells, a maximum demand pool for satisfying the maximum channel requirements of the cells, and a community pool for satisfying extraordinary demand during anomalous events. The first and second pools are each divided into seven subpools for allocation to the corresponding cells within each 7-cell cluster. The community pool is not divided according to cell type. Channels are allocated in order of preference from the base demand pool, the maximum demand pool and the community pool. A dynamic allocation scheme is applied where the static scheme is unable to satisfy a demand in a specific cell. However, the first and second pools cannot be assigned flexibly to accommodate asymmetry in demand, while the community pool cannot be assigned with spectral efficiency because it is not divided according to cell type.